1. Field of the Invention
The present invention relates to a holographic recording medium.
2. Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, so-called page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Page-wise systems involve the storage and readout of an entire two-dimensional representation, e.g., a page of data. Typically, recording light passes through a two-dimensional array of dark and transparent areas representing data, and the holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index imprinted into a storage medium. Holographic systems are discussed generally in Psaltis et al., “Holographic Memories,” Scientific American, November 1995.
One method of holographic storage is phase correlation multiplex holography, which is described in U.S. Pat. No. 5,719,691 (Curtis et al.), issued Feb. 17, 1998. In one embodiment of phase correlation multiplex holography described in Curtis et al., a reference light beam is passed through a phase mask, and intersected in the recording medium with a signal beam that has passed through an array representing data, thereby forming a hologram in the medium. The spatial relation of the phase mask and the reference beam is adjusted for each successive page of data, thereby modulating the phase of the reference beam and allowing the data to be stored at overlapping areas in the medium. The data is later reconstructed by passing a reference beam through the original storage location with the same phase modulation used during data storage. It is also possible to use volume holograms as passive optical components to control or modify light directed at the medium, e.g., filters or beam steerers. Writing processes that provide refractive index changes are also capable of forming articles such as waveguides.
The capabilities of typical holographic recording systems are determined in part by the storage medium. One type of holographic recording media used recently for such systems are photosensitive polymer films. See, e.g., Smothers et al., “Photopolymers for Holography,” SPIE OE/Laser Conference, 1212-03, Los Angeles, Calif., 1990. The holographic recording media described in this article contain a photoimageable system containing a liquid monomer material (the photoactive monomer) and a photoinitiator (which promotes the polymerization of the monomer upon exposure to light), where the photoimageable system is in an organic polymer host matrix that is substantially inert to the exposure light. During writing (recording) of information into the material (by passing recording light through an array representing data), the monomer polymerizes in the exposed regions. Due to the lowering of the monomer concentration caused by the polymerization, monomer from the dark, unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting diffusion create a refractive index change, thus forming the hologram (holographic grating) representing the data.
Generally, in photopolymer systems used in conventional applications such as coatings, sealants, adhesives, etc., properties such as chain length and degree of polymerization are usually maximized and driven to completion by using very high light intensities, multifunctional monomers, high concentrations of monomers, heat, etc. Similarly, prior holographic recording media have used formulations that are higher in monomer concentration (as in typical photopolymer formulations) to provide holographic recording media based on organic photopolymer systems. See, for example, U.S. Pat. No. 5,874,187 (Colvin et al.), issued Feb. 23, 1999, and U.S. Pat. No. 5,759,721 (Dhal et al.), issued Jun. 2, 1998, which disclose what are often referred to as “one-component” organic photopolymer systems. Such one-component systems typically have large Bragg detuning values if they are not precured with light to some extent. Further improvements in holographic photopolymer media have also been made by separating the formation of the polymeric matrix from the photochemistry used to record holographic information. See, for example, U.S. Pat. No. 6,103,454 (Dhar et al.), issued Aug. 15, 2000, and commonly assigned, U.S. Pat. No. 6,482,551 (Dhar et al.), issued Nov. 19, 2002, which disclose what are often referred to as “two-component” organic photopolymer systems. Two-component organic photopolymer systems allow for more uniform starting conditions (i.e., regarding the recording process), more convenient processing and packaging options, and the ability to obtain very high dynamic range media with very little shrinkage or Bragg detuning.
Thus, even though large improvements in holographic media have been made, further improvements in such media would be desirable to: (1) preserve the specified pattern of holographic gratings in the media to allow for reliable retrieval of the recorded data in such media; (2) allow for appropriate scheduling in forming holographic gratings in the media; (3) more accurately determine the time needed for recording additional holographic gratings in the same volume of previously recorded media, up to the full dynamic range thereof; and (4) in general, to create more commercially viable high density holographic data storage media.